1. Field of the Invention
This invention relates to ovens, and more specifically, to ovens using independently controlled radiant heaters, both position and levels of heat output, with the optional combining of different methods of heating including ambient, convection, and radiant heat.
2. Related Art
Conventional curing ovens typically use one of three methods of heat transfer: convection, ambient, or radiant. Regarding the use of radiant heaters, such curing ovens typically use only one radiant heater which is locked in a fixed position. That is, the radiant heater is not dynamically movable to adjust the direction of the radiant heat emitted from the heater nor is the level of heat output from the radiant heater dynamically controlled. This results in an inefficient means for curing an object because the maximum effect of the heater cannot be customized or adjusted for different shaped objects being cured or for objects combining different compositions.
For example, in U.S. Pat. No. 5,836,085 to Ben-Ezra, an improved paint-spraying and curing booth is disclosed having direct fired U-tube radiant heaters. The radiant heaters are built into the radiant heating system, thereby being fixed in position. The radiant heaters are not movable or adjustable. Therefore, regardless of the size or shape of an object placed within the paint-spraying and curing oven, the radiant heaters will emit heat in the same direction and at the same angle every time. The same problem exists with the ovens disclosed in U.S. Pat. No. 5,588,830 to Josefsson, et al., and in U.S. Pat. No. 5,205,273 to Sparks, et al.
Therefore, there is a need for an oven having one or more radiant heaters that are dynamically movable such that the radiant heaters can be adjusted to maximize the efficiency of the emitted heat according to the size and/or shape of an object being cured/heated within the oven. There is a further need for an oven in which the levels of heat output from the radiant heaters can be dynamically controlled during the curing/heating process.
In addition to using fixed radiant heaters, a conventional curing oven may also combine two different methods of heat transfer, but there does not exist a curing oven that combines all three methods of heating. In the xe2x80x2830 Patent to Josefsson, et al., a combined radiant and convention heating oven is disclosed wherein the convection air is controlled independent of the radiant heat surfaces. The convection air is circulated via a plurality of fans that are controlled by a plurality of frequency motor drive units. The temperature of the radiating surfaces are controlled by heated air flowing through ducts extending longitudinally through the oven that abut the radiating surfaces. Optionally, dampers may be incorporated into the ducts to provide individual control to each radiating surface. An important feature of the Josefsson oven is the placement and use of a temperature sensor. The temperature sensor is placed in a return plenum to determine the convection air temperature. This temperature is then input into a control device which uses the convection temperature to control the speed of the fans and the amount of fuel supplied to the burners.
There are several disadvantages to the Josefsson heating oven, but the principal problem is the temperature sensor. By placing the temperature sensor in a location, e.g., the return plenum, for determining the convection air temperature, the object being cured can easily be either under-baked or over-baked. When an object, having bolts or the other components attached thereto, is under-baked, the paint will flake when a bolt is removed. When an object is over-baked, the paint becomes discolored.
Therefore, there is a need for a curing oven that combines ambient, convection, and radiant methods of heating that ensures a constant and uniform temperature of the object during the curing process.
The present invention is an oven having independently movable radiant heaters, wherein the levels of heat output from the radiant heaters also are dynamically controlled, and that optionally combines three types of heat: convection, ambient, and radiant. The oven comprises one or more adjustable U-Tube radiant heaters on each side of the internal oven chamber. The radiant heaters can pivot horizontally toward the object being heated as well as pivot vertically to direct the focus of the heaters, thereby taking into account the shape and size of the object. One or more back panels, preferably colored flat black, are positioned behind the radiant heaters to capture any radiant energy emitted from the radiant heaters that misses the object.
The oven of the present invention is directed to a curing oven being used for the powder based paint curing of an object; however, this is for convenience purpose only. The oven of the present invention can be adapted for use in pre-baking, baking, or drying-off an object, or in any other oven application requiring the heating of an object.
In the present invention, the oven can discharge the heat exhaust from the radiant heaters to either outside of the oven or back within the oven chamber, depending on the object being heated and the type of heating application, to help reach a maximum ambient temperature. This results in achieving ambient temperatures of 300-500 degrees much faster.
A conventional turbulent fan may be positioned on the top or side of the oven that works in combination with an air direction unit and a means for deflecting the airflow to circulate air within the oven chamber. The preferred means for deflecting airflow is one or more deflectors positioned along the walls of the oven chamber. The use of deflectors is beneficial to the heating process because they reduce dust build-up and are easier to clean than conventional duct work.
The radiant heaters, turbulent fan (convection heat), and ambient temperature (managed via the radiant heaters, exhaust fan and back panels) are all controlled by a computer system connected to one or more temperature sensors monitoring the temperature of the object being cured and the ambient temperature within the oven chamber. There are many advantages to the oven of the present invention. By dynamically controlling the position, direction, and level of heat (turning on and off) of the radiant heaters, the time needed to cure/heat an object is greatly reduced. The cure time is also improved due to the combination of convection, ambient, and radiant methods of heat. The following table illustrates sample test data for curing 14 gauge steel according to the present invention:
A curing oven of the present invention also requires much less fuel during operation. For example, a thirty (30) minute test using the present invention requires only 1/10th of a cubic foot of gas to maintain a proper cure. In addition, the curing oven of the present invention can be used with either powder based or water based paint. The curing oven also eliminates the majority of the EPA and OSHA problems associated with conventional spray painting of objects.
The oven of the present invention also maximizes the use of radiant heaters by allowing the radiant heaters the ability to pivot toward the object being cured or heated. In the preferred embodiment, the radiant heaters can pivot to within 6xe2x80x3 of the object as well as pivot vertically the direction of the emitted heat. The position of the radiant heaters also are adjusted dynamically during the entire curing/heating process, thereby ensuring proper curing at all times.
Furthermore, this pivoting of the radiant heaters and dynamic control of the level of heat from the radiant heaters provides the means for proportionally curing or heating an object from top to bottom. That is, the heating of an object can be customized according to the composition of the object. To properly cure an object, one portion, e.g., the bottom, may require slower cure time than another portion, e.g., the top, of the object, such as if the two portions were made from different gauges of steel. Therefore, to enable the object to be cured within the same time frame, thereby ensuring a proper and uniform cure, the radiant heaters may be controlled such that the bottom portion of the object cures at a lower temperature (and slower) than the top portion. In this example, one or more radiant heaters may be farther away from the bottom portion of the object and directed to a lower level of heat, whereas one or more other radiant heaters may be moved in closer to the top portion of the object and directed to a higher level of heat, resulting in the top and bottom portions of the object completing the cure process at the same time.
Lastly, one or more temperature sensors of the present invention is used to measure the temperature of the object being cured as well as the ambient temperature within the oven chamber. This is an important distinction over the prior art in that the prior art typically measures only the ambient temperature within the curing oven, and the temperature of an object being cured is always higher than the ambient and convection temperatures. Therefore, conventional curing ovens improperly heats an object resulting in the object being overbaked or underbaked. In contrast, by measuring the temperature of the object being cured, the object will never be incorrectly cured.